British ministers have rallied around the embattled foreign secretary Boris Johnson throughout Sunday, claiming that he was doing a “great job” and that despite his ill-considered remarks about the jailed Iranian-British aid worker Nazanin Zaghari-Ratcliffe which have potentially increased her sentence he should not resign.

The coordinated defence is part of attempts to shore up the government of prime minister Theresa May, weakened by a series of scandals and gaffes involving her top team of ministers as she negotiates Britain's departure from the European Union. The Sunday Times reported that 40 of her own MPs were now calling for her to step down.

Labour party leader Jeremy Corbyn called on May to sack Johnson, writing in The Observer newspaper on Sunday that “we’ve put up with him embarrassing and undermining our country through his incompetence ... for long enough. It’s time for Boris Johnson to go.”

Corbyn, and fellow Labourite Sadiq Khan, the mayor of London, said Johnson had offended states and religions before “bungling” the case of Iranian-British aid worker Mrs Zaghari-Ratcliffe, who is serving five years in an Iranian prison after being convicted of plotting to overthrow Iran’s clerical establishment. She denies the charges.

Two of Mr Johnson’s allies, Brexit minister David Davis and environment minister Michael Gove, defended the foreign secretary, who last week said he could have been clearer in his remark that Mrs Zaghari-Ratcliffe had been teaching people journalism before her arrest in April 2016.

Her employer, the Thomson Reuters Foundation, said Johnson’s comment made on November 1 was incorrect, while opposition MPs said the remarks could land the aid worker a longer term in jail.